1. Field of the Invention
The present invention relates to a particulate measurement system which measures the amount of particulates such as soot contained in a gas under measurement.
2. Description of the Related Art
Conventionally, a particulate measurement system has been known which measures the amount of particulates (e.g., soot) contained in a gas under measurement (for example, exhaust gas discharged from an internal combustion engine or the like) (see, for example, Patent Document 1).
Such a particulate measurement system includes a particulate sensor which is exposed to the gas under measurement and detects particulates. The particulate sensor includes an ion generation section, an electrification chamber, a trapping section, and an auxiliary electrode section.
The particulate measurement system electrifies at least a portion of particulates contained in the gas under measurement in the electrification chamber using ions generated at the ion generation section by means of corona discharge, and measures the amount of particulates based on the amount of ions trapped by the trapping section. Notably, the auxiliary electrode section is maintained at a potential which repulses the ions to thereby assist the trapping of ions at the trapping section.
Also, the particulate measurement system includes an isolation transformer for corona discharge and a corona discharge control section. The isolation transformer for corona discharge is an isolation transformer for voltage conversion of electric power used for corona discharge and includes a primary-side coil and a secondary-side coil which is connected to the ion generation section. The corona discharge control section detects a current flowing from the trapping section to a secondary-side reference potential (the reference potential of the secondary-side coil of the isolation transformer for corona discharge). Also, the corona discharge control section controls the amount of electric power supplied from the isolation transformer for corona discharge to the ion generation section based on the current such that the amount of ions generated from the ion generation section approaches a target value determined in advance.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2013-195069
3. Problems to be Solved by the Invention
However, a problem arises in the particulate measurement system described above in that when particulates (soot, etc.) adhere to the particulate sensor, the accuracy in measuring the amount of particulates is lowered or measurement of the amount of particulates can no longer be carried out.
Namely, for example, when the insulation resistance between the auxiliary electrode section and the reference potential (ground potential) decreases due to particulates, etc., having adhered to the particulate sensor, an irregular current may flow through the adhered particulates, etc. If such an irregular current flows, the measured amount of ions trapped by the trapping section is in error, which adversely affects the measurement of the amount of particulates. Also, if an irregular current is generated and the irregular current flows to a path extending from the trapping section to the secondary-side reference potential, the detected value of the current flowing from the trapping section to the secondary-side reference potential is an error. Further, the control of the amount of electric power by the corona discharge control section deviates from a normal operating state. In such a case, the amount of electric power supplied from the isolation transformer for corona discharge to the ion generation section deviates from a normal operating state and the accuracy in measuring the amount of particulates may decrease.